1. Field of the Invention
The present invention relates to radio frequency (RF) test probes, and particularly to a test probe arrangement that facilitates repeatable and accurate test measurements of RF circuit boards in automated production testing.
2. Discussion of the Known Art
To perform reliable operating tests on wireless products during production runs, accurate RF measurement instrumentation must be interfaced with each production unit quickly and in a way that ensures proper matching between the instrumentation and each unit to be tested. False test readings due to a bad connection between the measurement instrumentation and the production units can lower production efficiency, if a unit that would perform properly in the field tests erroneously as a "fail". Moreover, customer dissatisfaction will result when a unit that is really out of spec but measures falsely under test as a "pass", is allowed to be sold.
Present cellular and PCS telephone handsets operate in frequency ranges from around 800 MHZ to above 1900 MHZ, and test procedures at these frequencies must be performed with due care. For example, RF and ground contacts between the test instrumentation and the units under test must not act to cause mismatches or significant RF power losses.
One known technique for testing the performance of a hand-held cellular wireless unit includes connecting a length of small-diameter "mini" coaxial cable at one end to an antenna port of the unit, and the other end of the cable is fixed with other test leads to a multi-pin test connector mounted on the unit. This arrangement has certain disadvantages. For example, the length of "mini" cable acts to de-tune or load down the antenna port, and thus compromise the operating parameters established for the unit. Also, the cable does not present a matched load at the antenna port so that inaccurate test measurements can result.
Another known procedure calls for testing a RF circuit board by positioning it on a number of spring-loaded contact pins. A number of the pins contact ground conductors at various locations on the board, and these pins are connected in common to the body of a coaxial connector. A different pin contacts a board RF terminal and the board is pressed on the contact pins. The RF contact pin is connected to a center pin of the coaxial connector, and a test measurement instrument is connected by a cable to the connector. The arrangement fails to work properly at cellular and PCS radio frequencies, because (1) the spring loaded pins have lengths that look inductive to RF currents through them, (2) the multiple ground contacts with the circuit board cause undesirable RF loop currents, and (3) the impedance at the RF test point on the circuit board and that of the test instrument are not properly matched with one another.
A need therefore exists for a RF test probe arrangement for production RF equipment, which arrangement allows the equipment to be tested repeatedly and accurately for its performance when fully assembled and used in the field by consumers. Ideally, the probe arrangement should enable repeatable measurements of high volume production RF equipment at the circuit board level, and consistent with the actual performance of the equipment when fully assembled. At a minimum, the arrangement should allow test measurements of signal-in-noise-and-distortion to noise-and-distortion (SINAD) for analog units, and of bit-error rate (BER) for digital RF equipment; and it should not be susceptible to wear or require significant maintenance.